Humans have only one Insulin gene whereas the mouse has 2 insulin genes. In the pancreas the differentiation of endocrine cells is non-synchronous and lasts from the secondary transition period (which is the major time of endocrine cell differentiation) to the postnatal period. Unlike the mouse pancreas (in which beta cells are in direct contact with the blood vessel endothelium), in the human pancreas, beta cells are not in direct contact with the endothelium since they secrete a layer of extracellular matrix which acts as a second layer of basement membrane.